The incidence of death and debility due to cardiovascular diseases increases markedly in post-menopausal women. Although the effects of estrogen on lipoprotein metabolism have received considerable attention, little is known about the actual mechanisms through which ovarian hormones exert their wide-ranging and protective vascular effects. The long-term goal of these studies is to understand the cellular pathways through which ovarian steroids regulate resistance artery diameter, a principal determinant of total peripheral resistance and, therefore, systemic blood pressure. Our unifying hypothesis is that ovarian steroids, particularly estrogen, reduce arterial tone by altering the mechanisms involved in the transduction of physical forces (intravascular pressure and flow) by the cells (endothelium, smooth muscle) of the arterial wall. By taking advantage of small artery in vitro methodology that allows precise control of both physical forces and extracellular environment, the specific aims are to quantitatively assess the effects of ovarian function, its loss (surgical menopause), and hormonal reinstitution (estrogen replacement therapy) on endothelial secretion of vasoactive factors in response to transmural pressure and flow (Aim 1), the interaction of myogenic and adrenergic mechanisms in the control of lumen diameter (Aim 2), and on vascular smooth muscle cell calcium sensitivity and its modulation by G proteins and protein kinase C (Aim 3). Several innovative experimental approaches, including the perfusion of two vessels in series, permeabilization of the cell membrane with a bacterial exotoxin, and the use of selective pharmacological interventions will be applied under conditions in which transmural pressure, lumen diameter and flow are simultaneously measured and recorded. By understanding how ovarian steroids modulate mechano-transduction by the vascular wall, we hope to gain new insights into the mechanisms through which female sex hormones lower peripheral resistance and protect against cardiovascular disease.